Method of improving the ion selectivity of membranes

ABSTRACT

The invention features an electrochemical cell having two fluid-containing compartments separated by a non-selective microporous membrane. Select ions which would normally pass through the membrane under the influence of an ionic field, are prevented from passing through the membrane by a polyelectrolyte which has migrated through the compartment fluid to the membrane. The polyelectrolyte acts as an ionic barrier to the passage of select ions, thus effectively increasing the ion-selective capability of the membrane and, hence, the coulombic efficiency of the electro-chemical cell.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. Ser. No. 128,822,filed Mar. 10, 1980 now U.S. Pat. No. 4,259,417.

FIELD OF THE INVENTION

This invention pertains to electrochemical cells, and more particularlyto a method for improving separators or ionic barriers for such cells.

BACKGROUND OF THE INVENTION

In the manufacture of current-producing electrochemical cells, such assecondary battery cells, a membrane separator is often needed betweenelectrode compartments. The membrane separator is required toselectively pass ions from one compartment to the other. Theseion-exchange membranes are usually quite expensive, and can be thelimiting factor in the cost of production.

Microporous membranes having no ion selectivity have not beensuccessfully substituted for the more expensive ion-selective membranes.Such substitutions generally result in drastic reductions in coulombicefficiencies and are unacceptable from the standpoint of cellperformance.

The invention seeks to provide a method of improving the ion selectivityof membranes, wherein a low-cost electrochemical cell with acceptablecoulombic efficiences can be fabricated.

The invention proposes to utilize low-cost, non-selective microporousmembranes as battery separators by increasing in situ their capabilityto pass only select ions, and to furthermore achieve this capability ina low-cost manner.

DISCUSSION OF THE PRIOR ART

The use of ion-selective membranes is well known in the art. Thesemembranes are generally used to obtain high-coulombic efficiencies,prevent dendritic growth, and prevent unwanted ionic migration withinelectrochemical cells, as set forth in the U.S. Patents to: E. S. Long,entitled: "Primary Cell"; U.S. Pat. No. 3,015,681; issued: Jan. 2, 1962;R. B. Hodgdon, Jr., entitled "Bifunctional Cation Exchange Membranes andTheir Use in Electrolytic Cells", U.S. Pat. No. 3,657,104, issued: Apr.18, 1972; and D. W. Sheibley, entitled: "Formulated Plastic Separatorsfor Soluble Electrode Cells", U.S. Pat. No. 4,133,941, issued: Jan. 9,1979.

It is also known to add various materials to electrolytes in batterycells to inhibit dendrite formation and improve their chargingcharacteristics, as discussed in the U.S. patents to: F. G. Will,entitled: "Dendrite-Inhibiting Electrolytic Solution and RechargeableAqueous Zinc-Halogen Cell Containing the Solution". U.S. Pat. No.4,074,028, issued: Feb. 14, 1978; and S. Ikari, entitled: "Lead StorageBattery Containing a Sulfonic Acid Substituted Naphthalene/FormaldehydeCondensation Product", U.S. Pat. No. 3,481,785, issued: Dec. 2, 1969.

The prior art is prolific with membranes and electrolyte additives forimproving the performance of battery cells, but nowhere is it suggestedthat the use of an electrolyte additive can inhibit or influence theionic migration across a nonselective membrane.

SUMMARY OF THE INVENTION

The invention teaches that a microporous nonselective membrane in anelectrochemical cell can be modified in situ by polyelectrolytematerials disposed in the cell fluid, which materials migrate to themembrane surface. The polyelectrolytes are generally of a high molecularweight, and may have a generally convoluted shape wherein passage underthe influence of an ionic field through the micron-sized pores of themembrane is generally restricted. The pores of the membrane may likewisehave irregular or convoluted pathways to further restrict the passage ofthe polyelectrolytes.

Whether the polyelectrolytes actually penetrate the membrane or onlysubstantially coat the membrane surface is not well understood. However,it has been demonstrated that the polyelectrolytes will form a barrierto unwanted ions, and prevent their migration through the membrane. Theionic barrier is achieved by the physical restraint imposed upon thepolyelectrolyte molecules which have migrated to the membrane.

From another point of view, the polyelectrolyte material can beperceived as a means of ionically modifying the membrane in a selectivemanner in situ, i.e., modifying the selectivity characteristics of themembrane during operation of the cell.

A typical cell utilizing the invention will comprise at least twofluid-containing compartments separated by the nonselective microporousmembrane. To the fluid of one of the compartments is added anion-selective material, such as a polyelectrolyte of high molecularweight. Under the influence of an ionic field, the polyelectrolyte willmigrate toward the membrane where it will form an ionic barrier againstthe unwanted migration and passage of certain ones of the ions in thecompartmental fluid.

In a secondary battery cell supporting, for example, a zinc-bromidereaction, such induced ionic selectivity will improve the coulombicefficiency of the cell over that normally expected without the use ofthe polyelectrolyte.

The method of the invention seeks to improve the ion selectivity of allion-selective apparatuses by establishing a flow of ions in a fluidcontaining both ions and a polyelectrolyte. The polyelectrolyte tends tomigrate under the influence of the ionic flow and is then restrainedagainst migrating to form an ionically selective barrier to certain onesof the ions in the fluid.

It is an object of the invention to provide an improved method formodifying a barrier or separator for use in an electrochemical cell;

It is another object of this invention to provide a method of modifyingin situ a nonselective membrane to effectively produce an ion-selectivemembrane; and

It is a further object of this invention to provide a method of addingan additive to the electrolyte of an electrochemical cell which willform a selective ionic barrier when physically restrained from migratingwithin an ionic field of the cell.

These and other objects of this invention will become more apparent andwill be better understood with reference to the following detaileddescription considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a schematic diagram of an electrochemical cell comprising theinventive non-selective porous membrane and polyelectrolyte combinationfor selectively screening ions migrating in solution. The figure depictsthe polyelectrolyte in solution with an electric field being initiallyimpressed across the cell. The polyelectrolyte is shown beginning tomigrate towards the membrane;

FIG. 1b depicts the cell of FIG. 1 after the polyelectrolyte hasmigrated to the membrane and formed an ionic barrier;

FIG. 2 illustrates an alternate embodiment for the cell shown in FIG. 1;

FIG. 3 is a graph showing the improvement in coulombic efficiency withthe use of the invention; and

FIG. 4 is another graph illustrating the improvement in coulombicefficiency of an electrochemical bipolar cell with use of the inventivemethod.

DETAILED DESCRIPTION OF THE INVENTION

Generally speaking, the invention is for a method for modifying anapparatus to improve its ion selectivity. The apparatus generallycomprises a fluid containing ions and a flow of the ions is establishedin the fluid. A polyelectrolyte is disposed in the fluid and tends tomigrate under the influence of the ionic flow. The migration of thepolyelectrolyte is restrained such that an ionically selective barrieris formed by the polyelectrolyte. Certain ones of the ions in the fluidwill be screened from passing through the ionic barrier. The types ofions which can be screened by the polyelectrolyte will depend upon thecharge of the polyelectrolyte, i.e., negatively charged ions will bescreened by a negatively charged polyelectrolyte barrier; positivelycharged ions will be screened by a positively charged polyelectrolytebarrier; and screening of either positive or negative ions will beachieved with a barrier formed from an amphoterically chargedpolyelectrolyte at an appropriate pH.

For the purposes of this description, the term "polyelectrolyte" isgenerally defined as a substance of high molecular weight (generallygreater than 10,000) such as a long-chain polymer, a protein, amacromolecule, a polysaccharide, etc., which has a multiplicity of ionicsites.

The term "microporous membrane" is generally defined as a membranehaving a continuous pore network with an average pore size in the rangeof 0.005-0.30 microns.

It is contemplated that the present invention can be used in a widevariety of systems requiring ionic flow separation and selectivity, suchas Fuel Cells, Batteries, electrodialysis and water treatment systems,etc.

The ion-selective barrier can be used in systems wherein an electricfield is impressed across the cell, or an ionic fluid flows physicallythrough the cell or device.

Now referring to FIG. 1a, a simple electrochemical cell 10 comprisingthe invention is schematically illustrated. The cell 10 comprises acontainer 11 which is divided into two fluid-containing compartments 12and 13, respectively, by a nonselective microporous membrane 14. Thefluid 15 in each compartment can be the same or different. In one of thecompartments, such as compartment 13, the fluid 15 may contain negativeions 16.

The cell 10 has electrodes 17 and 18 for impressing an electric fieldacross the fluid 15 as shown by arrow 19.

The fluid 15 of compartment 13 contains negatively chargedpolyelectrolyte molecules 20.

When the electric field is impressed upon the cell, both the negativeions 16 and the polyelectrolyte molecules 20 will tend to migrate towardthe positive electrode 18 as shown by the arrows. The membrane 14 willnormally pass the negatively charged ions 16, but will not pass thepolyelectrolyte molecules 20 because of their size.

After a finite period of time, the polyelectrolyte molecules 20 willform a barrier layer 21 upon the membrane 14, as illustrated in FIG. 1b.Whether the polyelectrolyte molecules 20 actually penetrate the membranepores and get entangled therein due to the convoluted shape of themolecules, or are too big to penetrate the pores, is not fullyunderstood. What is known, however, is that a barrier is created within,or upon, the membrane 14. This barrier, being substantially of the sametype of charge (negative) as the ions 16, will tend to repel (arrows 22)these ions from passing through the membrane 14. In this manner, thepolyelectrolyte molecules 20 have now transformed the nonselectivity ofthe membrane 14 into a membrane having selectivity.

In another sense, the membrane 14 can be viewed as acting as a restraintfor the polyelectrolyte molecules 20 which form an ionic barrier 21 uponmigration to the membrane 14.

The polyelectrolyte molecules 20 can be positively charged wherepositive ions are meant to be prevented from passing through themembrane.

Both positive and negative ions can be prevented from passing throughthe membrane with the use of either an amphoterically chargedpolyelectrolyte or the use of both positive and negativepolyelectrolytes in one or more of the compartments 12 and 13,respectively.

Where the polyelectrolyte molecules 20 have a specific gravity greaterthan the fluid 15, they may tend to sink to the bottom of thecompartment 13 during operation or storage of the cell 10. To insurethat the polyelectrolyte is properly distributed or circulated in thefluid 15, a stirrer 25 can be utilized.

In other systems the fluid 15 containing the polyelectrolyte 20 can becirculated through the compartment 13 using a conduit (not shown)forming a closed loop feeding into and out of the compartment. Areservoir (not shown) in the closed loop will supply the conduit andcompartment with a fresh supply of fluid. A pump (not shown) can bedisposed in the conduit to effect the circulation throughout the closedloop.

Referring to FIG. 2, an alternate embodiment is schematicallyillustrated for the cell 10 of FIGS. 1a and 1b. A cell 10' comprises acontainer 31 with three separate fluid-containing compartments 32, 33,and 34, respectively. The fluid 35 may be the same or different in eachcompartment. An electric field can be established across the cell 10' bymeans of electrodes 37 and 38, respectively.

The cell 10' is divided into the three compartments 32,33, and 34,respectively, by means of two nonselective microporous membranes 36a and36b.

The second or middle compartment 33 contains the polyelectrolytematerial 20, such that ionic flow in either direction can be madeselective, i.e., flow from either compartment 32 to compartment 34,and/or vice versa.

EXAMPLE 1

The invention was tested in a Zn/Br₂ battery system of the type shown inthe U.S. Patent to: Agustin F. Venero, entitled: "Metal HalogenBatteries and Method of Operating Same"; U.S. Pat. No. 4,105,829;issued: Aug. 8, 1978, the description of which is meant to beincorporated herein by reference. A nine- (9) plate monopolar cell wasconstructed with ten (10) mil thick Daramic® membranes separating thecompartments. The Daramic® membranes Series HW-0835 were obtained fromW. R. Grace Company, Polyfibron Division, Cambridge, Mass. Thesemembranes are microporous and ionically nonselective and are the typegenerally used in automotive batteries. These membranes had an averagepore size of 0.05 microns (BET method), with a maximum pore size of 0.10microns. The average pore volume was 55±5%.

The coulombic efficiency of this electrochemical cell was tested withand without the use of a polyelectrolyte. The polyelectrolyte used inthe tests was a sulfonated polystyrene of about 70,000 molecular weightand known under the product name of Versa-TL 72-SD, made by NationalStarch, Bound Brook, N.J. The cell was run through forty (40) cycles asshown in FIG. 3. About 0.5 percent by weight of the Versapolyelectrolyte was added to the catholyte at the fifth (5th) cycle, andabout 0.15 percent by weight of the Versa polyelectrolyte was added atthe fifteenth (15th) cycle, as shown. At the twenty-third (23rd) andthirty-fifth (35th) cycles, the system was discharged at 1 amp., till anOCV of -1.7 volts was obtained, and then charged at 1 amp. till an OCVof +1.7 volts was acheived.

As can be observed from FIG. 3, the addition of the polyelectrolyteimproved the coulombic efficiency of the System from approximately 62%to approximately 80-85%.

The results of the test illustrated in FIG. 3 are also shown in Tabularform in Table I below.

                  TABLE I                                                         ______________________________________                                        NINE- (9) PLATE MONOPOLAR UNTREATED                                           DARAMIC® 10 MIL THICK FLAT SHEET                                          Cycle No.              Efficiency                                             ______________________________________                                         1                     70.7%                                                   2                     72.2                                                    3                     67.4                                                    4                     65.7                                                    5                     61.5                                                   add 0.50 wt % polyelectrolyte                                                  6                     67.8                                                    7                     73.7                                                    8                     76.2                                                   10                     81.9                                                   11                     82.6                                                   12                     84.2                                                   13                     82.4                                                   14                     83.9                                                   add 0.15 wt % polyelectrolyte catholyte                                       15                     80.0                                                   16                     83.3                                                   17                     83.0                                                   18                     82.1                                                   20                     76.8                                                   21                     79.7                                                   22                     80.6                                                   23                     76.8                                                   24                     79.8                                                   25                     82.5                                                   26                     80.4                                                   27                     80.9                                                   28                     78.3                                                   29                     80.7                                                   30                     79.1                                                   31                     80.6                                                   32                     79.4                                                   33                     77.5                                                   34                     77.8                                                   35                     76.9                                                   36                     81.2                                                   37                     82.2                                                   38                     80.8                                                   39                     82.3                                                   40                     78.6                                                   ______________________________________                                    

EXAMPLE 2

The above cell of Example 1 was again constructed as before, but nowusing microporous membranes of CELGARD-2400+2500, made by CelaneseCorp., having an average pore size of 0.02-0.4 microns. These membranesproduced a cell having a coulombic efficiency of approximately 50-55%.Addition of 0.15 weight% of the Versa polyelectrolyte increased thecoulombic efficiency to about 70%.

EXAMPLE 3

A bipolar Zn/Br₂ battery was constructed in accordance with theteachings of the U.S. Pat. No. 4,105,829, previously mentioned. Thebattery comprised eight (8) cells (12 V.) of approximately 6 dm².Daramic® membranes of 24 mil thickness were used but otherwise similarto those described in Example 1. The polyelectrolyte used was Versa-TL72-SD as mentioned before. The test procedure comprised acharge-discharge cycling of the battery. Charging was done at 11.6 Amps(20 mA/cm²) for 3 hours and 60 and 90 mA-h/cm² Zn loading for 41/2hours. Discharging was accomplished at 17.4 A (30 mA/cm²) to 8 V. Thefirst 25 cycles were run using an electrolyte of 3 M ZnBr₂ and 1.0 MN-ethyl, N-Methylmorpholinium Bromide: ##STR1##

The electrolyte was then changed to 3 M ZnBr₂ and 0.5 M N-ethyl,N-Methylmorpoholinium/0.5 M N-ethyl, N-methylpyrolidinium Bromide:##STR2##

FIG. 4 depicts the test results in terms of coulombic efficiency vs.cycle number.

The Versa-TL72-SD was added at cycle number 10. The coulombic efficiencywas noted to increase from about 72% without the polyelectrolyte toapproximately 80% with the addition of the polyelectrolyte.

In these examples, the coulombic efficiency of the Zn/Br₂ battery wasdecreased by self-discharge when bromine migrates to the zinc electrode.In solution, bromine exists as a negatively charged Br₃.sup.⊖ which isrepelled by negatively charged ion-selective membranes, therebyimproving coulombic efficiency.

The above examples are meant to be merely exemplary teachings of how theinvention may be practiced.

The microporous Daramic® membranes which have been found to work mostsatisfactorily in this invention have an average pore size of about 0.01to 0.06 microns, but other materials and pore sizes may be possible. Themembranes may be manufactured from a polypropylene or a polystyrene orother suitable polymer. Such membranes will generally comprise 30 to 90%void space.

The polyelectrolyte as used in cells of the general type describedherein, may be either a sulfonated or carboxylated polystyrene. Othercells will naturally require different polyelectrolyte materials.

Having thus described the invention, what is meant to be protected byLetters Patent is presented in the following appended claims.

What is claimed is:
 1. In an electrochemical cell, comprising: a firstfluid-containing compartment; a second fluid-containing compartment; anonselective porous membrane disposed between said first and secondfluid-containing compartments; A method of modifying the ion selectivityof said membrane comprising the steps of:(a) passing ions through saidmembrane; and (b) migrating a polyelectrolyte disposed in a fluid of atleast one of said compartments towards, and forming an ionic barrier at,said membrane in response to the ionic flow through said membrane, saidpolyelectrolyte modifying the ion selectivity of said membrane bycausing substantially only selected ions to pass through said membrane.2. The method of claim 1, wherein said polyelectrolyte is substantiallynegatively charged.
 3. The method of claim 1, wherein saidpolyelectrolyte is substantially positively charged.
 4. The method ofclaim 1, wherein said polyelectrolyte is amphoterically charged.
 5. Themethod of claim 1, wherein said polyelectrolyte contains a sulfonatedpolystyrene.
 6. The method of claim 1, wherein said polyelectrolytecontains a carboxylated polystyrene.
 7. The method of claim 1, whereinsaid membrane is microporous.
 8. The method of claim 1, furthercomprising the step of circulating fluid in at least one of saidfluid-containing compartments.
 9. In an electrochemical cell,comprising: a first fluid-containing compartment; a secondfluid-containing compartment; a third fluid-containing compartment; afirst nonselective porous membrane of a set disposed between said firstand second fluid-containing compartments; a second nonselective porousmembrane of said set disposed between said second and thirdfluid-containing compartments; the method of modifying the ionselectivity of said membranes comprising the steps of:(a) passing ionsthrough said membranes; (b) adding a polyelectrolyte to a fluid of saidsecond fluid-containing compartment; and (c) passing only selected ionsthrough said set of membranes.
 10. The method of claim 9, wherein saidpolyelectrolyte is substantially negatively charged.
 11. The method ofclaim 9, wherein said polyelectrolyte is substantially positivelycharged.
 12. The method of claim 9, wherein said polyelectrolyte isamphoterically charged.
 13. The method of claim 9, wherein saidpolyelectrolyte contains a sulfonated polystyrene.
 14. The method ofclaim 9, wherein said polyelectrolyte contains a carboxylatedpolystyrene.
 15. The method of claim 9, wherein said membranes aremicroporous.
 16. The method of claim 9, further comprising the step of:circulating a fluid in at least one of said fluid-containingcompartments.
 17. In an ion-selective apparatus, the method of improvingthe ion selectivity of said apparatus comprising the steps of:(a)establishing a flow of ions in a fluid containing both ions and apolyelectrolyte, said polyelectrolyte tending to migrate under theinfluence of said ionic flow; and (b) restraining said polyelectrolyteagainst migration in said fluid, whereby said polyelectrolyte will forman ionically selective barrier to certain ones of said ions in saidfluid.
 18. The method of claim 17, wherein said polyelectrolyte issubstantially negatively charged.
 19. The method of claim 17, whereinsaid polyelectrolyte is substantially positively charged.
 20. The methodof claim 17, wherein said polyelectrolyte is amphoterically charged. 21.The method of claim 17, wherein said polyelectrolyte contains asulfonated polystyrene.
 22. The method of claim 17, wherein saidpolyelectrolyte contains a carboxylated polystyrene.
 23. The method ofclaim 17 wherein the ion-selective apparatus is contained within abattery system.
 24. The method of claim 23, wherein the battery systemis rechargeable.
 25. The method of claim 24 wherein the battery systemis a zinc-bromine system.